The present invention relates to the field of jet drop printing and more particularly to an improved fluid jet print head and a method of operation therefor.
Jet drop printers operate by generating streams of small drops of ink and controlling the deposit of the drops on a print receiving medium. Typically, the drops are electrically charged and then deflected by an electrical field. The drops are formed from fluid filaments which emerge from small orifices. The orifices may be formed in an orifice plate which communicate with a fluid reservoir in which fluid is maintained under pressure. Each fluid filament tends to break apart at its tip to form a stream of drops. In order to produce accurate printing it is necessary that the drops be generated at accurately timed intervals. This is accomplished by a process known as "stimulation".
One prior art approach vibrates the entire print head, including the ink manifold structure and the orifice plate structure, together. This is shown in Beam et al U.S. Pat. No. 3,586,907. Such an arrangement necessarily fatigues the print head mounting structure, since the mounting structure experiences the same vibrations as are applied to the manifold and the orifice plate. Further, the amplitude and phase of the vibratory motion are difficult to control at the frequencies commonly used for jet drop printer operation.
Another prior art stimulation technique, as shown in Lyon et al U.S. Pat. No. 3,739,393, provides the fluid orifices in a relatively thin, flexible orifice plate. The orifice plate is stimulated by causing a series of bending waves to travel therealong. This technique, known as traveling wave stimulation, results in substantially uniform drop size and spacing, but the timing of break up of the fluid filaments varies along the length of the orifice plate.
Other prior art approaches have attempted to stimulate the filaments in a common phase by exciting coplanar movement of the orifices in the orifice plate, a typical example is disclosed in Cha U.S. Pat. No. 4,095,232. Using the technique disclosed in this patent, stimulators mounted in the upper portion of a fluid reservoir generate pressure waves which are transmitted downward through the fluid. Each stimulator includes a pair of piezoelectric crystals which vibrate in phase and which are mounted on opposite sides of a mounting plate which is coincident with a nodal plane. A reaction mass is positioned at the end of each stimulator opposite the stimulation member. The reaction mass ensures that the nodal plane is properly positioned.
In British Patent Specification No. 1,293,980, and Cha et al U.S. Pat. No. 4,198,643, print heads are disclosed in which a pair of piezoelectric crystals are bonded to opposite sides of a support plate. A print head manifold structure is bonded to one of the piezoelectric crystals and a counterbalance is bonded to the other of the crystals. The weight of the counterbalance is selected so as to offset the weight of the manifold structure. By this balanced arrangement, the support plate is placed in a nodal plane when the two piezoelectric transducers are energized in synchronism.
Finally, in Keur U.S. Pat. No. 3,972,474, an ink drop writing system is shown in which a vibrating nozzle is used to produce a stream of drops. The length of the nozzle is selected so that its mechanical resonant frequency is much higher than the frequency at which it is driven. The nozzle, configured as a tube, is surrounded by a piezoelectric ring which, when electrically driven, provides radial contraction and expansion of the tube.
Generally speaking, the prior art stimulation systems have employed piezoelectric crystals incorporated into mechanical arrangements of complex acoustical design. Each such arrangement has had to be individually tailored for resonant operation at the design frequency within its specifically associated print head. Such tailoring has required careful mechanical adjustment and/or trial and error selection of component parts. This has "tuned" the stimulation system for operation within an extremely narrow range of operating frequencies. For operation outside this range the performance is extremely degraded.
In some applications it is desirable to adjust the frequency of the stimulation driving signal. A typical example is in precision printing of high resolution graphics. In such printing there are unavoidable variations in the transport speed of the substrate, and these variations tend to produce drop positional placement errors. This can be corrected by adjusting the stimulation drive, as shown for instance in Van Brimer et al U.S. Pat. No. 3,588,906. This results in stimulation at a frequency which deviates from the nominal design frequency. Such deviation cannot be accomodated satisfactorily by systems of the above described types.
Thus it is seen that there is a need for an improved and simplified apparatus for effecting fluid jet stimulation and for accommodating adjustments in the frequency of the stimulation.